Prevent crash when Quad::arclen() returns 0
Prevent crash when Quad::arclen() returns 0

If Quad::arclen() is 0 then we end up dividing by zero which eventually
causes a crash in MeasureFunctor(). Not sure if this is a proper fix,
but it prevents the crash and the optimised glyph looks fine.

Fixes #6

googlefonts/fontcrunch,moyogo/fontcrunch,moyogo/fontcrunch,googlefonts/quadopt,googlefonts/fontcrunch,googlefonts/quadopt